This invention relates to the measurement of laminar flow over airfoils and more particularly to the determination of the existence and location of crossflow vorticity disturbances and transition to turbulence on laminar flow surfaces.
The accurate determination of both the location and cause of boundary-layer transition from laminar to turbulent flow is important in basic research for validation of theory and design. A determination of the locations of laminar and turbulent flow on the surfaces of wings, empennage, fuselage, and engine nacelles aids in this basic research. The transition to turbulence of three dimensional laminar boundary layers such as on swept wings or nonaxisymmetric bodies can be caused by any individual or combination of six different instabilities or disturbances. These six modes of instabilities or disturbances are inflectional instability in the free-shear layer across a laminar separation bubble, acoustic disturbances, roughness or waviness of the flow surfaces, catastrophic amplification of two-dimensional wavelike (Tollimien-Schlichting) disturbances, attachment line contamination, and crossflow vorticity. Crossflow vorticity results from the combined effect of streamwise and spanwise pressure gradients in the boundary layer and can form three-dimensional vortical disturbances which grow to cause transition to turbulence.
Prior methods of determining the crossflow vorticity in laminar flow utilize visual, electronic, and optical techniques. First, the visual technique involves observing surface coatings such as oil and sublimating chemicals to provide distinctive streamwise streaks indicative of the crossflow vorticity and the ensuing transition front. However, low temperatures (about -20.degree. C.) of flight tests conducted at high altitudes (above approximately 20,000 feet) render sublimating chemicals useless. Also, neither oil flow nor sublimating chemicals can provide several data points for a given test flight or wind tunnel run. Also, the locations of maximum amplitude of the crossflow vorticity are not readily apparent using these visualization methods. Thus, if data are required over a range of test speeds or angles of attack, the visual techniques are expensive and time-consuming. Second, the electronic technique involves mounting multiple hot-wire probes on the surface. These hot wire probes extend into the boundary layer, causing a local transition wedge to form in the downstream direction. Thus, measurements made in the chordwise direction require a longitudinal traversing mechanism or will require staggered sensors to avoid contamination of a downstream probe by an upstream probe. Hot wire probes are also inappropriate for many flight tests. Mounting a longitudinal traversing mechanism on a wing for flight can entail unacceptable costs and complexity. Also, many holes would have to be drilled in the wing for support. This is impractical for many test airplanes. The necessary hot wire sensing elements would need to be small. Airframe vibrations increase the likelihood of failure of these fragile elements. Also, since the hot wire probes would be large in comparison with crossflow vorticity patterns, good spatial resolution of the pattern of crossflow vorticity wavelengths may not be obtainable. Third, the optical technique involves laser velocimetry to measure crossflow vorticity. This technique includes large and expensive equipment. Also, the flow must be "seeded" with microscopic particles of materials such as latex rubber, alcohol, or clay particles, thereby excluding this optical technique from some wind tunnel tests and from practical flight tests.
Accordingly, it is an object of this invention to detect crossflow vorticity both electronically and non-invasively.
It is a further object of this invention to detect crossflow vorticity resulting from variable freestream conditions during a single flight test or wind tunnel run.
A further object of this invention is to detect the spatial (i.e., wavelength) and time-dependent behavior (e.g., frequency of non-stationary instability) of crossflow vorticity.
A further object of this invention is to measure crossflow vorticity in a simple, cheap, and spatially efficient manner.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawing which follow.